The present invention relates to amplitude modulated lasers, and more particularly to a tracking modulation limiter for lasers to provide control of the depth of modulation.
Semiconductor laser diodes have two distinct modes or areas of operation that are functions of the forward current flowing in them. The small current light emitting diode (LED) region is characterized by low differential (slope) efficiency of non-coherent radiant flux. The large current region is characterized by high efficiency of coherent radiant flux. The transition region where the semiconductor diode begins to "lase" is termed the threshold. When amplitude modulating a laser diode it is desirable to remain above threshold to maintain a detectable optical carrier, yet it is also desirable to allow modulation peaks to approach threshold to maintain as large a dynamic range as possible. Amplitude modulation can be either white positive or white negative modulation. In television transmission white negative modulation is often employed because the synchronizing information is then transmitted at maximum power. Further the laser diode transform characteristics change as a function of temperature and age which varies the threshold.
What is desired is a tracking modulation limiter for laser diodes which limits the negative modulation that can be applied to a laser diode to assure that the laser diode remains above threshold while maintaining as large a dynamic range as possible regardless of variations in the threshold due to temperature and age of the laser diode.